[NTI] Teach TypeICompilerTestCase to run testExternChanges in both OTI and NTI.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168072759

Author: shicks

test/com/google/javascript/jscomp/TypeICompilerTestCase.java

@@ -88,6 +88,20 @@ protected void testInternal(
     }
   }
 
+  @Override
+  protected void testExternChanges(String extern, String input, String expectedExtern,
+      DiagnosticType... warnings) {
+    if (this.mode.runsOTI()) {
+      testExternChangesOTI(extern, input, expectedExtern, warnings);
+    }
+    if (this.mode.runsNTI()) {
+      testExternChangesNTI(extern, input, expectedExtern, warnings);
+    }
+    if (this.mode.runsNeither()) {
+      super.testExternChanges(extern, input, expectedExtern, warnings);
+    }
+  }
+
   // Note: may be overridden to allow different externs if necessary.
   void checkMinimalExterns(Iterable<SourceFile> externs) {
     try {
@@ -134,6 +148,26 @@ private void testNTI(
     this.mode = saved;
   }
 
+  private void testExternChangesOTI(String extern, String input, String expectedExtern,
+      DiagnosticType... warnings) {
+    TypeInferenceMode saved = this.mode;
+    this.mode = TypeInferenceMode.OTI_ONLY;
+    enableTypeCheck();
+    super.testExternChanges(extern, input, expectedExtern, warnings);
+    disableTypeCheck();
+    this.mode = saved;
+  }
+
+  private void testExternChangesNTI(String extern, String input, String expectedExtern,
+      DiagnosticType... warnings) {
+    TypeInferenceMode saved = this.mode;
+    this.mode = TypeInferenceMode.NTI_ONLY;
+    enableNewTypeInference();
+    super.testExternChanges(extern, input, expectedExtern, warnings);
+    disableNewTypeInference();
+    this.mode = saved;
+  }
+
   void testWarningOtiNti(
       String js, DiagnosticType otiWarning, DiagnosticType ntiWarning) {
     TypeInferenceMode saved = this.mode;