fix(analyzer): fix a warning about an unused variable

Author: vicb

modules/change_detection/src/parser/context_with_variable_bindings.js

@@ -31,7 +31,7 @@ export class ContextWithVariableBindings {
   }
 
   clearValues() {
-    for (var [k, v] of MapWrapper.iterable(this.varBindings)) {
+    for (var k of MapWrapper.keys(this.varBindings)) {
       MapWrapper.set(this.varBindings, k, null);
     }
   }

modules/facade/src/collection.dart

@@ -50,6 +50,8 @@ class MapWrapper {
   static void delete(m, k) { m.remove(k); }
   static void clear(m) { m.clear(); }
   static Iterable iterable(m) { return new IterableMap(m); }
+  static Iterable keys(m) { return m.keys; }
+  static Iterable values(m) { return m.values; }
 }
 
 // TODO: how to export StringMap=Map as a type?

modules/facade/src/collection.es6

@@ -25,6 +25,8 @@ export class MapWrapper {
   static delete(m, k) { m.delete(k); }
   static clear(m) { m.clear(); }
   static iterable(m) { return m; }
+  static keys(m) { return m.keys(); }
+  static values(m) { return m.values(); }
 }
 
 // TODO: cannot export StringMap as a type as Dart does not support renaming types...