dart2js: Replace semicolons with newlines in minified output

sdk/lib/_internal/compiler/implementation/js/printer.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.
 
 part of js;
 
 class Printer implements NodeVisitor {
   final bool shouldCompressOutput;
   leg.Compiler compiler;
   leg.CodeBuffer outBuffer;
   int indentLevel = 0;
   bool inForInit = false;
   bool atStatementBegin = false;
   final DanglingElseVisitor danglingElseVisitor;
   final LocalNamer localNamer;
   bool pendingSemicolon = false;
   bool pendingSpace = false;
   static final identifierCharacterRegExp = new RegExp(r'^[a-zA-Z_0-9$]');
+  static final expressionContinuationRegExp = new RegExp(r'^[-+([]');
 
   Printer(leg.Compiler compiler, { allowVariableMinification: true })
       : shouldCompressOutput = compiler.enableMinification,
         this.compiler = compiler,
         outBuffer = new leg.CodeBuffer(),
         danglingElseVisitor = new DanglingElseVisitor(compiler),
         localNamer = determineRenamer(compiler.enableMinification,
                                       allowVariableMinification);
 
   static LocalNamer determineRenamer(bool shouldCompressOutput,
@@ skipping 16 matching lines @@
 
   String lastAddedString = null;
   int get lastCharCode {
     if (lastAddedString == null) return 0;
     assert(lastAddedString.length != "");
     return lastAddedString.charCodeAt(lastAddedString.length - 1);
   }
 
   void out(String str) {
     if (str != "") {
-      if (pendingSemicolon && (!shouldCompressOutput || str != "}")) {
-        outBuffer.add(";");
+      if (pendingSemicolon) {
+        if (!shouldCompressOutput) {
+          outBuffer.add(";");
+        } else if (str != "}") {
+          // We want to output newline instead of semicolon because it makes
+          // the raw stack traces much easier to read and it also makes line-
+          // based tools like diff work much better.  JavaScript will
+          // automatically insert the semicolon at the newline if it means a
+          // parsing error is avoided, so we can only do this trick if the
+          // next line is not something that can be glued onto a valid
+          // expression to make a new valid expression.
+          if (expressionContinuationRegExp.hasMatch(str)) {
+            outBuffer.add(";");
+          } else {
+            outBuffer.add("\n");
+          }
+        }
       }
       if (pendingSpace &&
           (!shouldCompressOutput || identifierCharacterRegExp.hasMatch(str))) {
         outBuffer.add(" ");
       }
       pendingSpace = false;
       pendingSemicolon = false;
       outBuffer.add(str);
       lastAddedString = str;
     }
@@ skipping 1019 matching lines @@
         codes.add(nthLetter((n ~/ nameSpaceSize) % LETTERS));
       }
       codes.add(charCodes.$0 + digit);
       newName = new String.fromCharCodes(codes);
     }
     assert(new RegExp(r'[a-zA-Z][a-zA-Z0-9]*').hasMatch(newName));
     maps.last[oldName] = newName;
     return newName;
   }
 }