Delete a bunch of packages that are now on GitHub.

pkg/source_span/CHANGELOG.md

-# 1.0.2
-
-* Avoid unintentionally allocating extra objects for internal `FileSpan`
-  operations.
-
-* Ensure that `SourceSpan.operator==` works on arbitrary `Object`s.
-
-# 1.0.1
-
-* Use a more compact internal representation for `FileSpan`.
-
-# 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
-  inconsistent in its choice of which parameters were considered relative and
-  which absolute.