Add support to trace warnings in TraceBuffer.

runtime/vm/trace_buffer_test.cc

Index: runtime/vm/trace_buffer_test.cc
===================================================================
--- runtime/vm/trace_buffer_test.cc	(revision 37220)
+++ runtime/vm/trace_buffer_test.cc	(working copy)
@@ -104,4 +104,30 @@
 }
 
 
+TEST_CASE(TraceBufferTraceWarning) {
+  Isolate* isolate = Isolate::Current();
+  TraceBuffer::Init(isolate, 3);
+  TraceBuffer* trace_buffer = isolate->trace_buffer();
+  const String& url = String::Handle(isolate, String::New("Plug"));
+  const String& source = String::Handle(isolate, String::New("240V"));
+  const Script& script = Script::Handle(isolate,
+      Script::New(url, source, RawScript::kScriptTag));
+  const intptr_t token_pos = 0;
+  const char* message = "High Voltage";
+  TraceBuffer::TraceWarningF(isolate, script, token_pos, "%s", message);
+  {
+    JSONStream js;
+    trace_buffer->PrintToJSONStream(&js);

[Cutch, 2014/06/11 21:45:03]

For testing, we could add accessors:

At(intptr_t i) const;
intptr_t Length() const;

to TraceBuffer.

[regis, 2014/06/11 23:48:35]

Done.

+    EXPECT_SUBSTRING("{\"type\":\"TraceBuffer\",\"members\":["
+                     "{\"type\":\"TraceBufferEntry\",\"time\":",
+                     js.ToCString());
+    // Skip time.
+    EXPECT_SUBSTRING("\"message\":\"{\\\"type\\\":\\\"TraceBufferWarning\\\","
+                     "\\\"script_url\\\":\\\"Plug\\\",\\\"token_pos\\\":0,"

[Cutch, 2014/06/11 21:45:03]

Please also add a test that exercises PrintToJSONStream

[regis, 2014/06/11 23:48:35]

Done.

+                     "\\\"message\\\":\\\"High Voltage\\\"\"}]}",
+                     js.ToCString());
+  }
+  delete trace_buffer;
+}
+
 }  // namespace dart