Avoid mangling the name of native methods.

frog/element.dart

Index: frog/element.dart
diff --git a/frog/element.dart b/frog/element.dart
index f041e3b3b4d64e39df54a317001d033b8de5ae7f..6f6a1223d3c2bd83c85dc093cd483bb2fd2d0749 100644
--- a/frog/element.dart
+++ b/frog/element.dart
@@ -18,7 +18,11 @@ class Element implements Hashable {
   Element _enclosingElement;
 
   Element(this.name, this._enclosingElement) {
-    _jsname = world.toJsIdentifier(name);
+    if (name !== null) {
+      String mangled = mangleJsName();
+      assert(!mangled.contains(':'));
+      _jsname = mangled;
+    }
   }
 
   // TODO - walk tree
@@ -51,6 +55,12 @@ class Element implements Hashable {
   void resolve() {}
 
   /**
+   * By default we mangle the JS name of an element to avoid
+   * giving them names that clash with JS keywords.
+   */
+  String mangleJsName() => world.toJsIdentifier(name);
+
+  /**
    * Any type parameters that this element defines to setup a generic
    * type resolution context.  This is currently used for both generic
    * types and the semi-magical generic factory methods - but it will