add checks needed for covariant generics, and List<E>

lib/runtime/dart/_js_helper.js

Index: lib/runtime/dart/_js_helper.js
diff --git a/lib/runtime/dart/_js_helper.js b/lib/runtime/dart/_js_helper.js
index 977407d40fd397404206155f4b86bb10d1123d37..aa9a7acfc5c0bc6971c52fc27d1431d44c86807a 100644
--- a/lib/runtime/dart/_js_helper.js
+++ b/lib/runtime/dart/_js_helper.js
@@ -41,12 +41,17 @@ var _js_helper;
         throw new core.UnsupportedError("Cannot modify unmodifiable Map");
       }
       set(key, val) {
+        dart.as(key, K);
+        dart.as(val, V);
         return this[_throwUnmodifiable]();
       }
       putIfAbsent(key, ifAbsent) {
+        dart.as(key, K);
+        dart.as(ifAbsent, dart.functionType(V, []));
         return dart.as(this[_throwUnmodifiable](), V);
       }
       remove(key) {
+        dart.as(key, K);
         return dart.as(this[_throwUnmodifiable](), V);
       }
       clear() {
@@ -73,6 +78,7 @@ var _js_helper;
         super._();
       }
       containsValue(needle) {
+        dart.as(needle, V);
         return this.values[core.$any](value => dart.equals(value, needle));
       }
       containsKey(key) {
@@ -113,6 +119,7 @@ var _js_helper;
   let ConstantProtoMap$ = dart.generic(function(K, V) {
     class ConstantProtoMap extends ConstantStringMap$(K, V) {
       _(length, jsObject, keys, protoValue) {
+        dart.as(protoValue, V);
         this[_protoValue] = protoValue;
         super._(dart.as(length, core.int), jsObject, dart.as(keys, core.List$(K)));
       }
@@ -164,6 +171,7 @@ var _js_helper;
         return this.$map;
       }
       containsValue(needle) {
+        dart.as(needle, V);
         return this[_getMap]().containsValue(needle);
       }
       containsKey(key) {