Fix an off-by-one bug between stack_trace and source_maps.

lib/source_map_stack_trace.dart

 // Copyright (c) 2015, 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.
 
 library source_map_stack_trace;
 
 import 'package:path/path.dart' as p;
 import 'package:source_maps/source_maps.dart';
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 33 matching lines @@
   var sdkLib = sdkRoot == null ? null : "$sdkRoot/lib";
 
   var trace = new Trace.from(stackTrace);
   return new Trace(trace.frames.map((frame) {
     // If there's no line information, there's no way to translate this frame.
     // We could return it as-is, but these lines are usually not useful anyways.
     if (frame.line == null) return null;
 
     // If there's no column, try using the first column of the line.
     var column = frame.column == null ? 0 : frame.column;
-    var span = sourceMap.spanFor(frame.line, column);
+
+    // Subtract 1 because stack traces use 1-indexed lines and columns and
+    // source maps uses 0-indexed.
+    var span = sourceMap.spanFor(frame.line - 1, column - 1);
 
     // If we can't find a source span, ignore the frame. It's probably something
     // internal that the user doesn't care about.
     if (span == null) return null;
 
     var sourceUrl = span.sourceUrl.toString();
     if (packageRoot != null && p.url.isWithin(packageRoot, sourceUrl)) {
       sourceUrl = "package:" +
           p.url.relative(sourceUrl, from: packageRoot);
     } else if (sdkRoot != null && p.url.isWithin(sdkLib, sourceUrl)) {
@@ skipping 33 matching lines @@
       .replaceAll(new RegExp(r"[a-zA-Z_0-9]+\$"), "")
       // Get rid of the static method prefix. The class name also exists in the
       // invocation, so we're not getting rid of any information.
       .replaceAll(new RegExp(r"^[a-zA-Z_0-9]+.static."), "")
       // Convert underscores after identifiers to dots. This runs the risk of
       // incorrectly converting members that contain underscores, but those are
       // contrary to the style guide anyway.
       .replaceAllMapped(new RegExp(r"([a-zA-Z0-9]+)_"),
           (match) => match[1] + ".");
 }