Split KernelToElementMap into sub-interfaces

pkg/compiler/lib/src/js_model/js_strategy.dart

Index: pkg/compiler/lib/src/js_model/js_strategy.dart
diff --git a/pkg/compiler/lib/src/js_model/js_strategy.dart b/pkg/compiler/lib/src/js_model/js_strategy.dart
index 74474516e964827154a941a744533004c153e122..5b457a4951bf773e59e80fa27ac757bac8b1ac56 100644
--- a/pkg/compiler/lib/src/js_model/js_strategy.dart
+++ b/pkg/compiler/lib/src/js_model/js_strategy.dart
@@ -36,16 +36,16 @@ class JsBackendStrategy implements KernelBackendStrategy {
   final JsToFrontendMap _map = new JsToFrontendMapImpl();
   ElementEnvironment _elementEnvironment;
   CommonElements _commonElements;
-  KernelToElementMap _elementMap;
+  KernelToElementMapForBuilding _elementMap;
   ClosureConversionTask _closureDataLookup;
   final GlobalLocalsMap _globalLocalsMap = new GlobalLocalsMap();
 
   JsBackendStrategy(this._compiler);
 
-  KernelToElementMap get elementMap {
+  KernelToElementMapForBuilding get elementMap {
     if (_elementMap == null) {
       KernelFrontEndStrategy strategy = _compiler.frontendStrategy;
-      KernelToElementMap elementMap = strategy.elementMap;
+      KernelToElementMapForBuilding elementMap = strategy.elementMap;
       _elementMap = new JsKernelToElementMap(
           _map, _elementEnvironment, _commonElements, elementMap);
     }