Allow window.Math to be folded to just Math

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124744949

src/com/google/javascript/jscomp/PeepholeSubstituteAlternateSyntax.java

@@ -118,11 +118,12 @@ public Node optimizeSubtree(Node node) {
     }
   }
 
-  private static final ImmutableSet<String> BUILTIN_TYPES = ImmutableSet.of(
+  private static final ImmutableSet<String> BUILTIN_EXTERNS = ImmutableSet.of(
       "Object",
       "Array",
       "Error",
-      "RegExp");
+      "RegExp",
+      "Math");
 
   private Node tryMinimizeWindowRefs(Node node) {
     // Normalization needs to be done to ensure there's no shadowing. The window prefix is also
@@ -139,7 +140,7 @@ private Node tryMinimizeWindowRefs(Node node) {
 
       // Since normalization has run we know we're referring to the global window.
       if ("window".equals(nameNode.getString())
-          && BUILTIN_TYPES.contains(stringNode.getString())) {
+          && BUILTIN_EXTERNS.contains(stringNode.getString())) {
         Node newNameNode = IR.name(stringNode.getString());
         Node parentNode = node.getParent();
 

test/com/google/javascript/jscomp/PeepholeSubstituteAlternateSyntaxTest.java

@@ -261,6 +261,7 @@ public void testRemoveWindowRefs() {
     fold("x = window.Array", "x = Array");
     fold("x = window.Error", "x = Error");
     fold("x = window.RegExp", "x = RegExp");
+    fold("x = window.Math", "x = Math");
 
     // Not currently handled by the pass but should be folded in the future.
     foldSame("x = window.String");