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

pkg/source_maps/lib/source_maps.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+/// Source maps library.
+///
+/// Create a source map using [SourceMapBuilder]. For example:
+///     var json = (new SourceMapBuilder()
+///         ..add(inputSpan1, outputSpan1)
+///         ..add(inputSpan2, outputSpan2)
+///         ..add(inputSpan3, outputSpan3)
+///         .toJson(outputFile);
+///
+/// Use the [Span] and [SourceFile] classes to specify span locations.
+///
+/// Parse a source map using [parse], and call `spanFor` on the returned mapping
+/// object. For example:
+///     var mapping = parse(json);
+///     mapping.spanFor(outputSpan1.line, outputSpan1.column)
+library source_maps;
+
+export "builder.dart";
+export "parser.dart";
+export "printer.dart";
+export "span.dart";