Add widget to display errors, and report dependency_graph errors correctly

lib/runtime/messages.css

Index: lib/runtime/messages.css
diff --git a/lib/runtime/messages.css b/lib/runtime/messages.css
new file mode 100644
index 0000000000000000000000000000000000000000..0f99a9017823e56bd0523917b92847c8349142d1
--- /dev/null
+++ b/lib/runtime/messages.css
@@ -0,0 +1,78 @@
+/** Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file

[Siggi Cherem (dart-lang), 2015/03/07 02:43:01]

this CSS was copied from the widget that we have in polymer, it's identical to
what Jake and I had originally written

+    for details. All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file. **/
+.dev-compiler-messages {
+  position: fixed;
+  bottom: 0;
+  right: 0;
+  max-width: 80vw;
+  z-index: 10000;
+  font-family: sans-serif !important;
+}
+.dev-compiler-messages .message {
+  padding: 0.6em;
+  background: black;
+  color: white;
+  border: solid 1px #666;
+  border-bottom: 0px;
+  text-overflow: ellipsis;
+  overflow-x: hidden;
+}
+.dev-compiler-messages .fine {
+  color: green;
+}
+.dev-compiler-messages .info {
+  color: yellow;
+}
+.dev-compiler-messages .warning {
+  color: orange;
+}
+.dev-compiler-messages .severe {
+  color: red;
+}
+.dev-compiler-messages div.text {
+  white-space: nowrap;
+  cursor: pointer;
+}
+.dev-compiler-messages div.text.expanded {
+  white-space: normal;
+}
+.dev-compiler-messages div.text a {
+  color: #CCF;
+  text-decoration: bold;
+}
+.dev-compiler-messages div.menu {
+  text-align: right;
+}
+.dev-compiler-messages .menu div {
+  display: inline-block;
+  background: #666;
+  font-weight: bold;
+  cursor: pointer;
+  border: solid 1px black;
+  padding: 0.6em 1em;
+}
+.dev-compiler-messages .menu div.active {
+  background: black;
+}
+.dev-compiler-messages .menu div .num {
+  color: white;
+}
+.dev-compiler-messages .content {
+  max-height: 75vh;
+  font-size: 1em;
+  overflow-y: auto;
+}
+.dev-compiler-messages .content > div {
+  display: none;
+}
+.dev-compiler-messages .content > div.active {
+  display: block;
+}
+
+.dev-compiler-messages .content span.text {
+  padding: 0.4em 0.2em 0.2em 2em;
+  white-space: pre;
+  display: block;
+  font-family: monospace !important;
+}