Reland: Add more API fuzzing and fix the issues encountered.

runtime/bin/string_stream.dart

Index: runtime/bin/string_stream.dart
diff --git a/runtime/bin/string_stream.dart b/runtime/bin/string_stream.dart
index 16ba1a7c3462fb72877a3d1225f4d4a5732d998c..cfbc37c982ac9e09050245d7574a64faee39de06 100644
--- a/runtime/bin/string_stream.dart
+++ b/runtime/bin/string_stream.dart
@@ -37,7 +37,11 @@ class _StringDecoders {
     } else if (encoding == Encoding.ASCII) {
       return new _AsciiDecoder();
     } else {
-      throw new StreamException("Unsupported encoding ${encoding.name}");
+      if (encoding is Encoding) {
+        throw new StreamException("Unsupported encoding ${encoding.name}");
+      } else {
+        throw new StreamException("Unsupported encoding ${encoding}");
+      }
     }
   }
 }