dart2js: Match the runtime int is check semantics at compile time.

pkg/compiler/lib/src/js_backend/constant_system_javascript.dart

Index: pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
index 290d563259dd393b4b5d0bda51b1008d7ca2f710..6c57cf3854807af7e7a0d78f7874c3573999b28b 100644
--- a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
@@ -275,7 +275,11 @@ class JavaScriptConstantSystem extends ConstantSystem {
   }
 
   // Integer checks don't verify that the number is not -0.0.

[asgerf, 2016/01/26 14:59:18]

Update doc comment please.

-  bool isInt(ConstantValue constant) => constant.isInt || constant.isMinusZero;
+  bool isInt(ConstantValue constant) {
+    return constant.isInt || constant.isMinusZero ||
+        constant.isPositiveInfinity ||
+        constant.isNegativeInfinity;
+  }
   bool isDouble(ConstantValue constant)
       => constant.isDouble && !constant.isMinusZero;
   bool isString(ConstantValue constant) => constant.isString;