Move source maps package to the dart repo, so it can be used by other internal

pkg/source_maps/README.md

+Source Maps
+===========
+
+This project implements a Dart pub package to work with source maps. The
+implementation is based on the [source map version 3 spec][spec] which was
+originated from the [Closure Compiler][closure] and has been implemented in
+Chrome and Firefox.
+
+In this package we provide:
+  * Data types defining file locations and spans: these are not part of the
+    original source map specification. These data types are great for tracking
+    source locations on source maps, but they can also be used by tools to
+    reporting useful error messages that include on source locations.
+  * A builder that creates a source map programatically and produces the encoded
+    source map format.
+  * A parser that reads the source map format and provides APIs to read the
+    mapping information.
+
+Some upcoming features we are planning to add to this package are:
+  * A printer that lets you generate code, but record source map information in
+    the process.
+  * A tool that can compose source maps together. This would be useful for
+    instance, if you have 2 tools that produce source maps and you call one with
+    the result of the other.
+
+[closure]: http://code.google.com/p/closure-compiler/wiki/SourceMaps
+[spec]: https://docs.google.com/a/google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit