Add breaking changes and release 1.0.0.

lib/src/span_scanner.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:source_span/source_span.dart';
 
 import 'eager_span_scanner.dart';
 import 'exception.dart';
 import 'line_scanner.dart';
 import 'relative_span_scanner.dart';
@@ skipping 20 matching lines @@
           "this LineScanner.");
     }
 
     this.position = state.position;
   }
 
   /// The [FileSpan] for [lastMatch].
   ///
   /// This is the span for the entire match. There's no way to get spans for
   /// subgroups since [Match] exposes no information about their positions.
-  FileSpan get lastSpan => _lastSpan;
+  FileSpan get lastSpan {
+    if (lastMatch == null) _lastSpan = null;
+    return _lastSpan;
+  }
   FileSpan _lastSpan;
 
   /// The current location of the scanner.
   FileLocation get location => _sourceFile.location(position);
 
   /// Returns an empty span at the current location.
   FileSpan get emptySpan => location.pointSpan();
 
   /// Creates a new [SpanScanner] that starts scanning from [position].
   ///
@@ skipping 43 matching lines @@
     return true;
   }
 
   void error(String message, {Match match, int position, int length}) {
     validateErrorArgs(string, match, position, length);
 
     if (match == null && position == null && length == null) match = lastMatch;
     if (position == null) {
       position = match == null ? this.position : match.start;
     }
-    if (length == null) length = match == null ? 1 : match.end - match.start;
+    if (length == null) length = match == null ? 0 : match.end - match.start;
 
     var span = _sourceFile.span(position, position + length);
     throw new StringScannerException(message, span, string);
   }
 }
 
 /// A class representing the state of a [SpanScanner].
 class _SpanScannerState implements LineScannerState {
   /// The [SpanScanner] that created this.
   final SpanScanner _scanner;
 
   final int position;
   int get line => _scanner._sourceFile.getLine(position);
   int get column => _scanner._sourceFile.getColumn(position);
 
   _SpanScannerState(this._scanner, this.position);
 }