Add support to trace warnings in TraceBuffer.

runtime/vm/exceptions.h

Index: runtime/vm/exceptions.h
===================================================================
--- runtime/vm/exceptions.h	(revision 37297)
+++ runtime/vm/exceptions.h	(working copy)
@@ -35,9 +35,16 @@
   // Report a Javascript compatibility warning at the call site given by
   // caller_frame. Note that a JavascriptCompatibilityError is thrown
   // if --warning_as_error is specified.
+  // Also calls TraceJSWarningV below.
   static void JSWarning(StackFrame* caller_frame, const char* format, ...)
       PRINTF_ATTRIBUTE(2, 3);
 
+  // Emit a Javascript compatibility warning to the current trace buffer.
+  static void TraceJSWarningF(const Script& script, intptr_t token_pos,
+                              const char* format, ...) PRINTF_ATTRIBUTE(3, 4);
+  static void TraceJSWarningV(const Script& script, intptr_t token_pos,
+                              const char* format, va_list args);
+
   static RawStacktrace* CurrentStacktrace();
 
   // Helpers to create and throw errors.