Observatory strong mode fixes: Add abstract Location.toUserString to Location.
Observatory strong mode fixes: Add abstract Location.toUserString to Location.

This is necessary because many call sites invoke toUserString without
first casting to one of the derived classes.

This should be safe since both derived classes (SourceLocation and
UnresolvedSourceLocation) contain a definition of toUserString.

Change-Id: I69eeda326883d87fc4e8cedd732881d250cae8cb
Reviewed-on: https://dart-review.googlesource.com/55909
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk