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

lib/runtime/dart/convert.js

Index: lib/runtime/dart/convert.js
diff --git a/lib/runtime/dart/convert.js b/lib/runtime/dart/convert.js
index 4e2423972ae577ef125aa45cf2ab11689ede029c..9ef5dd31c7a221f17de710377a5a5a2559478e8b 100644
--- a/lib/runtime/dart/convert.js
+++ b/lib/runtime/dart/convert.js
@@ -9,9 +9,11 @@ var convert;
       Codec() {
       }
       encode(input) {
+        dart.as(input, S);
         return this.encoder.convert(input);
       }
       decode(encoded) {
+        dart.as(encoded, T);
         return this.decoder.convert(encoded);
       }
       fuse(other) {
@@ -380,6 +382,7 @@ var convert;
         super.ChunkedConversionSink();
       }
       add(chunk) {
+        dart.as(chunk, T);
         this[_accumulated][core.$add](chunk);
       }
       close() {
@@ -395,6 +398,7 @@ var convert;
         this[_sink] = sink;
       }
       add(data) {
+        dart.as(data, T);
         return this[_sink].add(data);
       }
       close() {
@@ -414,6 +418,7 @@ var convert;
         this[_chunkedSink] = converter.startChunkedConversion(sink);
       }
       add(o) {
+        dart.as(o, S);
         return this[_chunkedSink].add(o);
       }
       addError(error, stackTrace) {
@@ -476,6 +481,7 @@ var convert;
         super.Converter();
       }
       convert(input) {
+        dart.as(input, S);
         return dart.as(this[_second].convert(this[_first].convert(input)), T);
       }
       startChunkedConversion(sink) {