Remove Collection, Collections and clean up List/Set/Queue implementations of retain/remove.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index f3ad005bb4a3d4bf89dbb780f0606550161dd401..69caf46baa9684b663541d7d759baa48dbfe3846 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -734,7 +734,7 @@ class JavaScriptBackend extends Backend {
    * the generic version that contains all possible type checks is
    * also stored in this map.
    */
-  final Map<String, Collection<ClassElement>> specializedGetInterceptors;
+  final Map<String, Set<ClassElement>> specializedGetInterceptors;
 
   /**
    * Set of classes whose methods are intercepted.
@@ -767,8 +767,7 @@ class JavaScriptBackend extends Backend {
         oneShotInterceptors = new Map<String, Selector>(),
         interceptedElements = new Map<SourceString, Set<Element>>(),
         rti = new RuntimeTypes(compiler),
-        specializedGetInterceptors =
-            new Map<String, Collection<ClassElement>>(),
+        specializedGetInterceptors = new Map<String, Set<ClassElement>>(),
         super(compiler, JAVA_SCRIPT_CONSTANT_SYSTEM) {
     emitter = disableEval
         ? new CodeEmitterNoEvalTask(compiler, namer, generateSourceMap)