Issue 3540. Better error message for string concatenation

compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java

Index: compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java b/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
index fefae91f65586d377747581f90126dfd16b327af..4cd0107aba415ffc1bd06589c0f3b67a07747f82 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
@@ -14,11 +14,13 @@ import com.google.dart.compiler.resolver.MethodNodeElement;
 public class DartBinaryExpression extends DartExpression {
 
   private final Token op;
+  private final int opOffset;
   private DartExpression arg1;
   private DartExpression arg2;
   private MethodNodeElement element;
 
-  public DartBinaryExpression(Token op, DartExpression arg1, DartExpression arg2) {
+  public DartBinaryExpression(Token op, int opOffset, DartExpression arg1, DartExpression arg2) {
+    this.opOffset = opOffset;
     assert op.isBinaryOperator() : op;
 
     this.op = op;
@@ -37,6 +39,13 @@ public class DartBinaryExpression extends DartExpression {
   public Token getOperator() {
     return op;
   }
+  
+  /**
+   * @return the character offset of the {@link #getOperator()} token.
+   */
+  public int getOperatorOffset() {
+    return opOffset;
+  }
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {