Context providers: return dummy values if env is undefined

Throwing if "env" is undefined makes it hard to unit test stacks that
use context providers. It requires dummy values for account and region,
which is boilerplate and non-intuitive.

Since we already have a concept of dummy context values, this change
short-circuits context resolution and returns the dummy values in case
the stack's env is undefined.

packages/@aws-cdk/core/lib/context.ts

@@ -24,7 +24,12 @@ export class ContextProvider {
     /**
      * Read a provider value, verifying it's a string
      */
-    public getStringValue(provider: string, scope: string[], args: string[]): string {
+    public getStringValue(provider: string, scope: undefined | string[], args: string[], defaultValue = ''): string {
+        // if scope is undefined, this is probably a test mode, so we just
+        // return the default value
+        if (!scope) {
+            return defaultValue;
+        }
         const key = colonQuote([provider].concat(scope).concat(args)).join(':');
         const value = this.context.getContext(key);
         if (value != null) {
@@ -35,13 +40,19 @@ export class ContextProvider {
         }
 
         this.stack.reportMissingContext(key, { provider, scope, args });
-        return '';
+        return defaultValue;
     }
 
     /**
      * Read a provider value, verifying it's a list
      */
-    public getStringListValue(provider: string, scope: string[], args: string[], defaultValue = ['']): string[] {
+    public getStringListValue(provider: string, scope: undefined | string[], args: string[], defaultValue = ['']): string[] {
+        // if scope is undefined, this is probably a test mode, so we just
+        // return the default value
+        if (!scope) {
+            return defaultValue;
+        }
+
         const key = colonQuote([provider].concat(scope).concat(args)).join(':');
         const value = this.context.getContext(key);
 
@@ -59,7 +70,7 @@ export class ContextProvider {
     /**
      * Helper function to wrap up account and region into a scope tuple
      */
-    public accountRegionScope(providerDescription: string): string[] {
+    public accountRegionScope(providerDescription: string): undefined | string[] {
         const stack = Stack.find(this.context);
         if (!stack) {
             throw new Error(`${providerDescription}: construct must be in a stack`);
@@ -69,8 +80,7 @@ export class ContextProvider {
         const region = stack.env.region;
 
         if (account == null || region == null) {
-            // tslint:disable-next-line:max-line-length
-            throw new Error(`${providerDescription}: requires account and region information, but ${stack.name} doesn't have an "env" defined`);
+            return undefined;
         }
 
         return [account, region];

packages/@aws-cdk/core/test/test.context.ts

@@ -50,7 +50,14 @@ export = {
         test.deepEqual(azs, 'abc');
 
         test.done();
-    }
+    },
+
+    'Return default values if "env" is undefined to facilitate unit tests'(test: Test) {
+        const stack = new Stack();
+        test.deepEqual(new AvailabilityZoneProvider(stack).availabilityZones, [ 'dummy1a', 'dummy1b', 'dummy1c' ]);
+        test.deepEqual(new SSMParameterProvider(stack).getString('foo'), '');
+        test.done();
+    },
 };
 
 function firstKey(obj: any): string {