Reimplement SourceFile.getLine using binary search.

lib/compiler/implementation/source_file.dart

 // Copyright (c) 2012, 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_file');
 
 #import('dart:math');
 
 #import('colors.dart', prefix: 'colors');
 
@@ skipping 21 matching lines @@
         if (index <= 0) break;
         starts.add(index);
       }
       starts.add(text.length + 1);
       _lineStarts = starts;
     }
     return _lineStarts;
   }
 
   int getLine(int position) {
-    // TODO(jimhug): Implement as binary search.
-    var starts = lineStarts;
-    for (int i=0; i < starts.length; i++) {
-      if (starts[i] > position) return i-1;
+    List<int> starts = lineStarts;
+    if (position < 0 || starts.last() <= position) {
+      throw 'bad position #$position in file $filename with '
+            'length ${text.length}.';
     }
-    throw 'bad position #$position in file $filename with '
-          'length ${text.length}.';
+    int first = 0;
+    int count = starts.length;
+    while (count > 1) {
+      int step = count ~/ 2;
+      int middle = first + step;
+      int lineStart = starts[middle];
+      if (position < lineStart) {
+        count = step;
+      } else {
+        first = middle;
+        count -= step;
+      }
+    }
+    return first;
   }
 
   int getColumn(int line, int position) {
     return position - lineStarts[line];
   }
 
   /**
    * Create a pretty string representation from a character position
    * in the file.
    */
@@ skipping 25 matching lines @@
       }
 
       for (; i < toColumn; i++) {
         buf.add(color('^'));
       }
     }
 
     return buf.toString();
   }
 }