Fix calculated kernel for multilanguage

- closes nteract#1292

lib/store/index.js

@@ -35,9 +35,9 @@ export class Store {
   @computed
   get kernel(): ?Kernel {
     if (!this.filePath) return null;
-    const kernel = this.kernelMapping.get(this.filePath);
-    if (!kernel || kernel instanceof Kernel) return kernel;
-    if (this.grammar) return kernel[this.grammar.name];
+    const kernelOrMap = this.kernelMapping.get(this.filePath);
+    if (!kernelOrMap || kernelOrMap instanceof Kernel) return kernelOrMap;
+    if (this.grammar) return kernelOrMap.get(this.grammar.name);
   }
 
   @computed

spec/store/index-spec.js

@@ -367,15 +367,17 @@ describe("Store", () => {
       expect(store.kernel).toBeUndefined();
     });
 
-    it("should return null if no kernel for file", () => {
+    it("should return the correct kernel for multilanguage files", () => {
       store.editor = { getPath: () => "foo.md" };
       const kernel = new Kernel(
         new KernelTransport({
           display_name: "Python 3",
           language: "python"
         })
       );
-      store.kernelMapping = new Map([["foo.md", { python: kernel }]]);
+      store.kernelMapping = new Map([
+        ["foo.md", new Map([["python", kernel]])]
+      ]);
       store.grammar = { name: "python" };
       expect(store.kernel).toEqual(kernel);
     });