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here's the crash report!

---- Minecraft Crash Report ----

WARNING: coremods are present:
Contact their authors BEFORE contacting forge

// But it works on my machine.

Time: 05/02/17 09:00 PM
Description: There was a severe problem during mod loading that has caused the game to fail

net.minecraftforge.fml.common.LoaderException: java.lang.NoClassDefFoundError: com/pixelmonmod/pixelmon/client/models/blocks/ModelMechanicalAnvil
at net.minecraftforge.fml.common.LoadController.transition(LoadController.java:162)
at net.minecraftforge.fml.common.Loader.initializeMods(Loader.java:739)
at net.minecraftforge.fml.client.FMLClientHandler.finishMinecraftLoading(FMLClientHandler.java:310)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:495)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:329)
at net.minecraft.client.main.Main.main(SourceFile:124)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
Caused by: java.lang.NoClassDefFoundError: com/pixelmonmod/pixelmon/client/models/blocks/ModelMechanicalAnvil
at com.pixelmonmod.pixelmon.client.ClientProxy.registerRenderers(ClientProxy.java:228)
at com.pixelmonmod.pixelmon.Pixelmon.init(Pixelmon.java:133)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:560)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:211)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:189)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:118)
at net.minecraftforge.fml.common.Loader.initializeMods(Loader.java:737)
... 10 more
Caused by: java.lang.ClassNotFoundException: com.pixelmonmod.pixelmon.client.models.blocks.ModelMechanicalAnvil
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 39 more
Caused by: java.lang.OutOfMemoryError: Java heap space
at java.util.HashMap.resize(HashMap.java:703)
at java.util.HashMap.putVal(HashMap.java:662)
at java.util.HashMap.put(HashMap.java:611)
at sun.net.www.protocol.jar.URLJarFile.getManifest(URLJarFile.java:155)
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:147)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at com.pixelmonmod.pixelmon.client.ClientProxy.registerRenderers(ClientProxy.java:228)
at com.pixelmonmod.pixelmon.Pixelmon.init(Pixelmon.java:133)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:560)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:211)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:189)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
Minecraft Version: 1.8.9
Operating System: Windows 7 (x86) version 6.1
CPU: 2x Intel(R) Celeron(R) CPU 1005M @ 1.90GHz
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 299440 bytes (0 MB) / 523501568 bytes (499 MB) up to 523501568 bytes (499 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.19 Powered by Forge 11.15.1.1902 Optifine OptiFine_1.8.9_HD_U_H6 4 mods loaded, 4 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
UCHI mcp{9.19} [Minecraft Coder Pack] (minecraft.jar)
UCHI FML{8.0.99.99} [Forge Mod Loader] (forge-1.8.9-11.15.1.1902-1.8.9.jar)
UCHI Forge{11.15.1.1902} [Minecraft Forge] (forge-1.8.9-11.15.1.1902-1.8.9.jar)
UCHE pixelmon{4.3.1} [Pixelmon] (Pixelmon-1.8.9-4.3.1-universal.jar)
Loaded coremods (and transformers):
GL info: ' Vendor: 'Intel' Version: '3.3.0 - Build 8.15.10.2696' Renderer: 'Intel(R) HD Graphics'
OptiFine Version: OptiFine_1.8.9_HD_U_H6
Render Distance Chunks: 8
Mipmaps: 4
Anisotropic Filtering: 1
Antialiasing: 0
Multitexture: false
Shaders: null
OpenGlVersion: 3.3.0 - Build 8.15.10.2696
OpenGlRenderer: Intel(R) HD Graphics
OpenGlVendor: Intel
CpuCount: 2

This is the wrong forum for support (See "Modded Client Support")

But to answer your issue,

Caused by: java.lang.OutOfMemoryError: Java heap space

means you ran out of memory and need to allocate more to minecraft to run this.

thanks a lot, sorry about that!