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

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

Index: mojo/public/dart/third_party/pub_semver/CHANGELOG.md
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+# 1.2.1
+
+* Allow version ranges like `>=1.2.3-dev.1 <1.2.3` to match pre-release versions
+  of `1.2.3`. Previously, these didn't match, since the pre-release versions had
+  the same major, minor, and patch numbers as the max; now an exception has been
+  added if they also have the same major, minor, and patch numbers as the min
+  *and* the min is also a pre-release version.
+
+# 1.2.0
+
+* Add a `VersionConstraint.union()` method and a `new
+  VersionConstraint.unionOf()` constructor. These each return a constraint that
+  matches multiple existing constraints.
+
+* Add a `VersionConstraint.allowsAll()` method, which returns whether one
+  constraint is a superset of another.
+
+* Add a `VersionConstraint.allowsAny()` method, which returns whether one
+  constraint overlaps another.
+
+* `Version` now implements `VersionRange`.
+
+# 1.1.0
+
+* Add support for the `^` operator for compatible versions according to pub's
+  notion of compatibility. `^1.2.3` is equivalent to `>=1.2.3 <2.0.0`; `^0.1.2`
+  is equivalent to `>=0.1.2 <0.2.0`.
+
+* Add `Version.nextBreaking`, which returns the next version that introduces
+  breaking changes after a given version.
+
+* Add `new VersionConstraint.compatibleWith()`, which returns a range covering
+  all versions compatible with a given version.
+
+* Add a custom `VersionRange.hashCode` to make it properly hashable.
+
+# 1.0.0
+
+* Initial release.