Support external implementations of FileSpan.

CHANGELOG.md

+# 1.1.3
+
+* `FileSpan.compareTo`, `FileSpan.==`, `FileSpan.union`, and `FileSpan.expand`
+  no longer throw exceptions for external implementations of `FileSpan`.
+
+* `FileSpan.hashCode` now fully agrees with `FileSpan.==`.

[Siggi Cherem (dart-lang), 2015/08/18 21:38:36]

I think before that was true too, maybe it's more that FileSpan.hashCode agrees
with SourceSpan.hashCode?

[nweiz, 2015/08/18 22:01:25]

Before, a FileSpan == a SourceSpan with the same start/end, but the hash codes
weren't equal.

+
 # 1.1.2
 
 * Fixed validation in `SourceSpanWithContext` to allow multiple occurrences of
   `text` within `context`.
 
 # 1.1.1
 
 * Fixed `FileSpan`'s context to include the full span text, not just the first
   line of it.
 
@@ skipping 44 matching lines @@
   `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
   inconsistent in its choice of which parameters were considered relative and
   which absolute.