Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/html/CHANGELOG.md

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+# changelog
+
+This file contains highlights of what changes on each version of the html
+package.
+
+#### Pub version 0.12.2
+  * Added `Element.endSourceSpan`, containing the span of a closing tag.
+
+#### Pub version 0.12.0+1
+  * Support `csslib` version `0.12.0`.
+
+#### Rename to package:html 0.12.0
+  * package has been renamed to `html`
+
+#### Pub version 0.12.0
+  * switch from `source_maps`' `Span` class to `source_span`'s
+    `SourceSpan` class.
+
+#### Pub version 0.11.0+2
+  * expand the version constraint for csslib.
+
+#### Pub version 0.10.0+1
+  * use a more recent source_maps version.
+
+#### Pub version 0.10.0
+  * fix how document fragments are added in NodeList.add/addAll/insertAll.
+
+#### Pub version 0.9.2-dev
+  * add Node.text, Node.append, Document.documentElement
+  * add Text.data, deprecate Node.value and Text.value.
+  * deprecate Node.$dom_nodeType
+  * added querySelector/querySelectorAll, deprecated query/queryAll.
+    This matches the current APIs in dart:html.