Remove usage of dart:json.

tools/coverage.dart

Index: tools/coverage.dart
diff --git a/tools/coverage.dart b/tools/coverage.dart
index 678fe5743010d3d4e78bc6c45f7a6db697002eff..34018c5482e4289546616507006cfd39b0ab80e0 100644
--- a/tools/coverage.dart
+++ b/tools/coverage.dart
@@ -15,7 +15,6 @@
 
 import "dart:convert";
 import "dart:io";
-import "dart:json" as JSON;
 
 
 // Whether or not to print debug target process on the console.
@@ -311,7 +310,7 @@ class Debugger {
         cleanup();
         return;
       }
-      var msgObj = JSON.parse(msg);
+      var msgObj = JSON.decode(msg);
       handleMessage(msgObj);
       if (errorsDetected) {
         error("Error while handling message from coverage target");
@@ -343,7 +342,7 @@ class Debugger {
   void sendMessage(Map<String,dynamic> msg) {
     assert(msg["id"] != null);
     msg["id"] = nextMessageId++;
-    String jsonMsg = JSON.stringify(msg);
+    String jsonMsg = JSON.encode(msg);
     if (verboseWire) print("SEND: $jsonMsg");
     socket.write(jsonMsg);
   }
@@ -481,7 +480,7 @@ class JsonBuffer {
   // buffer, or 0 if there is only a partial message in the buffer.
   // The object value must start with '{' and continues to the
   // matching '}'. No attempt is made to otherwise validate the contents
-  // as JSON. If it is invalid, a later JSON.parse() will fail.
+  // as JSON. If it is invalid, a later JSON.decode() will fail.
   int objectLength() {
     int skipWhitespace(int index) {
       while (index < buffer.length) {