Don't pass prop.name directly for error reporting

Author: JsonFreeman

src/compiler/checker.ts

@@ -8999,7 +8999,7 @@ module ts {
                             var typeName1 = typeToString(existing.containingType);
                             var typeName2 = typeToString(base);
 
-                            var errorInfo = chainDiagnosticMessages(undefined, Diagnostics.Named_property_0_of_types_1_and_2_are_not_identical, prop.name, typeName1, typeName2);
+                            var errorInfo = chainDiagnosticMessages(undefined, Diagnostics.Named_property_0_of_types_1_and_2_are_not_identical, symbolToString(prop), typeName1, typeName2);
                             errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Interface_0_cannot_simultaneously_extend_types_1_and_2, typeToString(type), typeName1, typeName2);
                             diagnostics.add(createDiagnosticForNodeFromMessageChain(typeNode, errorInfo));
                         }

tests/baselines/reference/symbolProperty35.errors.txt

@@ -0,0 +1,16 @@
+tests/cases/conformance/es6/Symbols/symbolProperty35.ts(8,11): error TS2320: Interface 'I3' cannot simultaneously extend types 'I1' and 'I2'.
+  Named property '[Symbol.toStringTag]' of types 'I1' and 'I2' are not identical.
+
+
+==== tests/cases/conformance/es6/Symbols/symbolProperty35.ts (1 errors) ====
+    interface I1 {
+        [Symbol.toStringTag](): { x: string }
+    }
+    interface I2 {
+        [Symbol.toStringTag](): { x: number }
+    }
+    
+    interface I3 extends I1, I2 { }
+              ~~
+!!! error TS2320: Interface 'I3' cannot simultaneously extend types 'I1' and 'I2'.
+!!! error TS2320:   Named property '[Symbol.toStringTag]' of types 'I1' and 'I2' are not identical.