Extract out a source_span package from source_maps.

pkg/source_span/CHANGELOG.md

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+# 1.0.0
+
+This package was extracted from the
+[`source_maps`](http://pub.dartlang.org/packages/source_maps) package, but the
+API has many differences. Among them:
+
+* `Span` has been renamed to `SourceSpan` and `Location` has been renamed to
+  `SourceLocation` to clarify their purpose and maintain consistency with the
+  package name. Likewise, `SpanException` is now `SourceSpanException` and
+  `SpanFormatException` is not `SourceSpanFormatException`.
+
+* `FixedSpan` and `FixedLocation` have been rolled into the `Span` and
+  `Location` classes, respectively.
+
+* `SourceFile` is more aggressive about validating its arguments. Out-of-bounds
+  lines, columns, and offsets will now throw errors rather than be silently
+  clamped.
+
+* `SourceSpan.sourceUrl`, `SourceLocation.sourceUrl`, and `SourceFile.url` now
+  return `Uri` objects rather than `String`s. The constructors allow either
+  `String`s or `Uri`s.
+
+* `Span.getLocationMessage` and `SourceFile.getLocationMessage` are now
+  `SourceSpan.message` and `SourceFile.message`, respectively. Rather than
+  taking both a `useColor` and a `color` parameter, they now take a single
+  `color` parameter that controls both whether and which color is used.
+
+* `Span.isIdentifier` has been removed. This property doesn't make sense outside
+  of a source map context.
+
+* `SourceFileSegment` has been removed. This class wasn't widely used and was

[Siggi Cherem (dart-lang), 2014/07/11 17:01:32]

I don't think we should remove this entirely. This is not used today in polymer
because we don't yet have support for piping source maps through barback, but
otherwise we would be using it.

In web_ui we use it to be able to parse the .dart code in an inlined <script>
tag inside html.

[nweiz, 2014/07/11 18:41:41]

As it exists currently in source_maps, it's not a good general-purpose API. It's
extremely inconsistent about what's treated as absolute vs relative to the base;
[location] takes a relative offset, but [getLine] and [getColumn] take absolute
offsets, for example.

If that API works for polymer, then maybe that's where the class should live,
but I don't think it's widely useful enough to be in this package.

+  inconsistent in its choice of which parameters were considered relative and
+  which absolute.