fix: export default in namespace AssemblyScript#2200

Author: technohippy

src/diagnosticMessages.json

@@ -114,6 +114,7 @@
   "A definite assignment assertion '!' is not permitted in this context.": 1255,
   "A class may only extend another class.": 1311,
   "A parameter property cannot be declared using a rest parameter.": 1317,
+  "A default export can only be used in an ECMAScript-style module.": 1319,
   "An identifier or keyword cannot immediately follow a numeric literal.": 1351,
 
   "Duplicate identifier '{0}'.": 2300,

src/program.ts

@@ -2565,6 +2565,13 @@ export class Program extends DiagnosticEmitter {
           this.initializeVariables(<VariableStatement>member, original);
           break;
         }
+        case NodeKind.EXPORT: {
+          this.error(
+            DiagnosticCode.A_default_export_can_only_be_used_in_an_ECMAScript_style_module,
+            member.range,
+          );
+          break;
+        }
         default: assert(false); // namespace member expected
       }
     }

tests/compiler/export-default-in-namespace.json

@@ -0,0 +1,7 @@
+{
+  "asc_flags": [
+   ],
+  "stderr": [
+    "TS1319: A default export can only be used in an ECMAScript-style module."
+  ]
+}

tests/compiler/export-default-in-namespace.ts

@@ -0,0 +1,3 @@
+namespace Foo {
+  export default foo;
+}