Fix for issue 3756

compiler/java/com/google/dart/compiler/parser/DartParser.java

Index: compiler/java/com/google/dart/compiler/parser/DartParser.java
===================================================================
--- compiler/java/com/google/dart/compiler/parser/DartParser.java	(revision 8906)
+++ compiler/java/com/google/dart/compiler/parser/DartParser.java	(working copy)
@@ -141,6 +141,7 @@
   private static final String INTERFACE_KEYWORD = "interface";
   private static final String NATIVE_KEYWORD = "native";
   private static final String NEGATE_KEYWORD = "negate";
+  private static final String ON_KEYWORD = "on";
   private static final String OPERATOR_KEYWORD = "operator";
   private static final String PREFIX_KEYWORD = "prefix";
   private static final String SETTER_KEYWORD = "set";
@@ -4113,6 +4114,7 @@
   }
 
   /**
+   * Parse either the old try statement syntax:
    * <pre>
    * tryStatement
    *     : TRY block (catchPart+ finallyPart? | finallyPart)
@@ -4126,6 +4128,24 @@
    *     : FINALLY block
    *     ;
    * </pre>
+   * or the new syntax:
+   * <pre>
+   * tryStatement
+   *     : TRY block (onPart+ finallyPart? | finallyPart)
+   *     ;
+   *
+   * onPart
+   *     : catchPart block
+   *     | ON qualified catchPart? block
+   *
+   * catchPart
+   *     : CATCH '(' identifier (',' identifier)? ')'
+   *     ;
+   *
+   * finallyPart
+   *     : FINALLY block
+   *     ;
+   * </pre>
    */
   private DartTryStatement parseTryStatement() {
     beginTryStatement();
@@ -4135,18 +4155,62 @@
     DartBlock tryBlock = parseBlock();
 
     List<DartCatchBlock> catches = new ArrayList<DartCatchBlock>();
-    while (optional(Token.CATCH)) {
+    while (peekPseudoKeyword(0, ON_KEYWORD) || match(Token.CATCH)) {
       // TODO(zundel): It would be nice here to setup 'FINALLY' as token for recovery
-      beginCatchClause();
-      expect(Token.LPAREN);
-      DartParameter exception = parseCatchParameter();
-      DartParameter stackTrace = null;
-      if (optional(Token.COMMA)) {
-        stackTrace = parseCatchParameter();
+      if (peekPseudoKeyword(0, ON_KEYWORD)) {
+        beginCatchClause();
+        next();
+        DartTypeNode exceptionType = new DartTypeNode(parseQualified());
+        DartParameter exception = null;
+        DartParameter stackTrace = null;
+        if (optional(Token.CATCH)) {
+          expect(Token.LPAREN);
+          beginCatchParameter();
+          DartIdentifier exceptionName = parseIdentifier();
+          exception = done(new DartParameter(exceptionName, exceptionType, null, null, Modifiers.NONE));
+          if (optional(Token.COMMA)) {
+            beginCatchParameter();
+            DartIdentifier stackName = parseIdentifier();
+            stackTrace = done(new DartParameter(stackName, null, null, null, Modifiers.NONE));
+          }
+          expectCloseParen();
+        } else {
+          // Create a dummy identifier that the user cannot reliably reference.
+          DartIdentifier exceptionName = new DartIdentifier("e" + Long.toHexString(System.currentTimeMillis()));
+          exception = new DartParameter(exceptionName, exceptionType, null, null, Modifiers.NONE);
+        }
+        DartBlock block = parseBlock();
+        catches.add(done(new DartCatchBlock(block, exception, stackTrace)));
+      } else {
+        beginCatchClause();
+        next();
+        expect(Token.LPAREN);
+        if (match(Token.IDENTIFIER) && (peek(1) == Token.COMMA || peek(1) == Token.RPAREN)) {
+          beginCatchParameter();
+          DartIdentifier exceptionName = parseIdentifier();
+          // Create a dummy type with the right semantics.
+          DartTypeNode exceptionType = new DartTypeNode(new DartIdentifier("Object"));
+          DartParameter exception = done(new DartParameter(exceptionName, exceptionType, null, null, Modifiers.NONE));
+          DartParameter stackTrace = null;
+          if (optional(Token.COMMA)) {
+            beginCatchParameter();
+            DartIdentifier stackName = parseIdentifier();
+            stackTrace = done(new DartParameter(stackName, null, null, null, Modifiers.NONE));
+          }
+          expectCloseParen();
+          DartBlock block = parseBlock();
+          catches.add(done(new DartCatchBlock(block, exception, stackTrace)));
+        } else {
+          DartParameter exception = parseCatchParameter();
+          DartParameter stackTrace = null;
+          if (optional(Token.COMMA)) {
+            stackTrace = parseCatchParameter();
+          }
+          expectCloseParen();
+          DartBlock block = parseBlock();
+          catches.add(done(new DartCatchBlock(block, exception, stackTrace)));
+        }
       }
-      expectCloseParen();
-      DartBlock block = parseBlock();
-      catches.add(done(new DartCatchBlock(block, exception, stackTrace)));
     }
 
     // Finally.