Add ModuleDescriptor.assertValid() for ensuring consistency

core/descriptors/src/org/jetbrains/kotlin/descriptors/ModuleDescriptor.kt

@@ -50,4 +50,6 @@ interface ModuleDescriptor : DeclarationDescriptor {
     }
 
     val isValid: Boolean
+
+    fun assertValid()
 }

core/descriptors/src/org/jetbrains/kotlin/descriptors/impl/ModuleDescriptorImpl.kt

@@ -48,8 +48,11 @@ class ModuleDescriptorImpl @JvmOverloads constructor(
     private var packageFragmentProviderForModuleContent: PackageFragmentProvider? = null
 
     override var isValid: Boolean = true
+        set(value) {
+            field = value
+        }
 
-    private fun assertValid() {
+    override fun assertValid() {
         if (!isValid) {
             throw IllegalStateException("Accessing invalid module descriptor $this")
         }

core/descriptors/src/org/jetbrains/kotlin/types/ErrorUtils.java

@@ -127,6 +127,11 @@ public class ErrorUtils {
             public boolean isValid() {
                 return false;
             }
+
+            @Override
+            public void assertValid() {
+                throw new IllegalStateException("ERROR_MODULE is not a valid module");
+            }
         };
     }