fix(jsii): prohibit exported const enums

'const enums' are inlined at the usage site by TypeScript and so the
generated type will not be in the JavaScript source code in the
assembly, even though the declaration will be there.

This leads to "symbol not found" errors upon trying to load it.

https://www.typescriptlang.org/docs/handbook/enums.html#const-enums

Fixes aws/aws-cdk#1969

packages/jsii/lib/assembler.ts

@@ -482,6 +482,15 @@ export class Assembler implements Emitter {
             LOG.trace(`Processing enum: ${colors.gray(namespace.join('.'))}.${colors.cyan(type.symbol.name)}`);
         }
 
+        const decl = type.symbol.valueDeclaration;
+        const flags = ts.getCombinedModifierFlags(decl);
+        // tslint:disable-next-line:no-bitwise
+        if (flags & ts.ModifierFlags.Const) {
+            this._diagnostic(decl,
+                             ts.DiagnosticCategory.Error,
+                            `Exported enum cannot be declared 'const'`);
+        }
+
         const jsiiType: spec.EnumType = {
             assembly: this.projectInfo.name,
             fqn: `${[this.projectInfo.name, ...namespace].join('.')}.${type.symbol.name}`,

packages/jsii/test/negatives/neg.const-enum.ts

@@ -0,0 +1,8 @@
+///!MATCH_ERROR: Exported enum cannot be declared 'const'
+
+export const enum NotAllowed {
+  ThisEnum,
+  GetsInlined,
+  AndSoItGetsLost,
+  ForJsii
+}