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

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

Index: mojo/public/dart/third_party/csslib/CHANGELOG.md
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+## 0.12.1
+
+ * Fix to handling of escapes in strings.
+
+## 0.12.0+1
+
+* Allow the lastest version of `logging` package.
+
+## 0.12.0
+
+* Top-level methods in `parser.dart` now take `PreprocessorOptions` instead of
+  `List<String>`.
+
+* `PreprocessorOptions.inputFile` is now final.
+
+## 0.11.0+4
+
+* Cleanup some ambiguous and some incorrect type signatures.
+
+## 0.11.0+3
+
+* Improve the speed and memory efficiency of parsing.
+
+## 0.11.0+2
+
+* Fix another test that was failing on IE10.
+
+## 0.11.0+1
+
+* Fix a test that was failing on IE10.
+
+## 0.11.0
+
+* Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan` class.