Keep only a single active stack trace, enables traces for thrown non-exceptions

pkg/dev_compiler/lib/js/es6/dart_sdk.js

Index: pkg/dev_compiler/lib/js/es6/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/es6/dart_sdk.js b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
index d71474d601c95219b0866dcdccce017a411e98b6..3a48f4865b4fc2b0166d7e84823d0dd6a7516b58 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -1897,14 +1897,12 @@ dart.assert = function(condition) {
   if (!condition) dart.throwAssertionError();
 };
 dart.throw = function(obj) {
-  if (obj != null && (typeof obj == 'object' || typeof obj == 'function')) {
-    dart._stack.set(obj, new Error());
-  }
+  dart._stack = new Error();
   throw obj;
 };
 dart.getError = function(exception) {
-  var stack = dart._stack.get(exception);
-  return stack !== void 0 ? stack : exception;
+  var stack = dart._stack;
+  return stack !== null ? stack : exception;
 };
 dart.stackPrint = function(exception) {
   var error = dart.getError(exception);
@@ -2662,7 +2660,7 @@ dart._ignoreTypeFailure = (() => {
     return false;
   });
 })();
-dart._stack = new WeakMap();
+dart._stack = null;
 dart._value = Symbol("_value");
 dart.constants = new Map();
 dart.constantLists = new Map();