Invalid argument when trying to parse '100 px'

[devoncarew]

With dart-archive/csslib#167, we introduced a bug where we throw an exception when trying to parse 100 px (as opposed to 100px).

Unhandled exception:
Invalid argument(s): Spans <_FileSpan: from <FileLocation: 145 unknown source:1:146> to <FileLocation: 148 unknown source:1:149> "100"> and <_FileSpan: from <FileLocation: 149 unknown source:1:150> to <FileLocation: 151 unknown source:1:152> "px"> are disjoint.
#0      _FileSpan.union (package:source_span/src/file.dart:378:9)
dart-lang/tools#747      _Parser.processDimension (package:csslib/parser.dart:2424:21)
dart-lang/tools#748      _Parser.processTerm (package:csslib/parser.dart:2400:11)
dart-lang/tools#749      _Parser.processExpr (package:csslib/parser.dart:2148:33)
dart-lang/tools#750      _Parser.processDeclaration (package:csslib/parser.dart:1760:19)
dart-lang/tools#751      _Parser.processDeclarations (package:csslib/parser.dart:1178:18)
dart-lang/tools#752      _Parser.processRule (package:csslib/parser.dart:1095:37)
dart-lang/tools#753      _Parser.parse (package:csslib/parser.dart:205:20)
dart-lang/tools#754      parse (package:csslib/parser.dart:99:32)
...