chore(grunt): reduce JVM heap size hint on win

The current grunt scripts require at least 2GB of heap space to be
allocated by the JVM, even though this is not needed to actually complete
the build. Adding 64-bit to the mix, this means grunt package fails on
machines without a whole 4GB of RAM free.

Closes angular#4595

Author: tathamoddie

lib/grunt/utils.js

@@ -172,7 +172,7 @@ module.exports = {
     shell.exec(
         'java ' +
             this.java32flags() + ' ' +
-            '-Xmx2g ' +
+            '-Xmx512m ' +
             '-cp bower_components/closure-compiler/compiler.jar' + classPathSep +
             'bower_components/ng-closure-runner/ngcompiler.jar ' +
             'org.angularjs.closurerunner.NgClosureRunner ' +