Adds a warning if the + opearator is used with two strings.

compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java

Index: compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
diff --git a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
index 719a9b20808cc5f957f8dbd84b0c1854d81e7b12..b2bf34ca799263e83b68b599a3ba85874053c6de 100644
--- a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
+++ b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
@@ -347,7 +347,10 @@ public class TypeAnalyzer implements DartCompilationPhase {
           return rhs;
         }
 
-        case ASSIGN_ADD:
+        case ASSIGN_ADD: {
+          checkStringConcatPlus(lhsNode);
+          checkStringConcatPlus(rhsNode);
+        }
         case ASSIGN_SUB:
         case ASSIGN_MUL:
         case ASSIGN_DIV:
@@ -405,7 +408,10 @@ public class TypeAnalyzer implements DartCompilationPhase {
           }
         }
 
-        case ADD:
+        case ADD:  {
+          checkStringConcatPlus(lhsNode);
+          checkStringConcatPlus(rhsNode);
+        }
         case SUB:
         case MUL:
         case DIV:
@@ -441,6 +447,22 @@ public class TypeAnalyzer implements DartCompilationPhase {
       }
     }
 
+    private void checkStringConcatPlus(DartExpression node) {  
+      if (node != null) {
+        Type type = null;
+        if (node.getElement() != null) { 
+          type = node.getElement().getType();
+        } else if (node.getType() != null) {
+          type = node.getType();
+        } 
+        if (type != null) {
+          if (type.equals(stringType)) {
+            onError(node, TypeErrorCode.PLUS_CANNOT_BE_USED_FOR_STRING_CONCAT);
+          }
+        }
+      }
+    }
+
     @Override
     public Type visitVariableStatement(DartVariableStatement node) {
       Type type = typeOf(node.getTypeNode());