Make the fix for #365 not be a breaking change (at least, for the considerable majority of aspnet-webpack users)

src/Microsoft.AspNetCore.SpaServices/npm/aspnet-webpack/src/WebpackDevMiddleware.ts

@@ -50,7 +50,13 @@ function attachWebpackDevMiddleware(app: any, webpackConfig: webpack.Configurati
             }
 
             // Now also inject eventsource polyfill so this can work on IE/Edge (unless it's already there)
+            // To avoid this being a breaking change for everyone who uses aspnet-webpack, we only do this if you've
+            // referenced event-source-polyfill in your package.json. Note that having event-source-polyfill available
+            // on the server in node_modules doesn't imply that you've also included it in your client-side bundle,
+            // but the converse is true (if it's not in node_modules, then you obviously aren't trying to use it at
+            // all, so it would definitely not work to take a dependency on it).
             const eventSourcePolyfillEntryPoint = 'event-source-polyfill';
+            if (npmModuleIsPresent(eventSourcePolyfillEntryPoint)) {
                 const entryPointsArray: string[] = entryPoints[entryPointName]; // We know by now that it's an array, because if it wasn't, we already wrapped it in one
                 if (entryPointsArray.indexOf(eventSourcePolyfillEntryPoint) < 0) {
                     const webpackHmrIndex = firstIndexOfStringStartingWith(entryPointsArray, webpackHotMiddlewareEntryPoint);
@@ -62,6 +68,7 @@ function attachWebpackDevMiddleware(app: any, webpackConfig: webpack.Configurati
                     // Insert the polyfill just before the HMR entrypoint
                     entryPointsArray.splice(webpackHmrIndex, 0, eventSourcePolyfillEntryPoint);
                 }
+            }
         });
 
         webpackConfig.plugins = [].concat(webpackConfig.plugins || []); // Be sure not to mutate the original array, as it might be shared
@@ -189,3 +196,12 @@ function firstIndexOfStringStartingWith(array: string[], prefixToFind: string) {
 
     return -1; // Not found
 }
+
+function npmModuleIsPresent(moduleName: string) {
+    try {
+        require.resolve(moduleName);
+        return true;
+    } catch (ex) {
+        return false;
+    }
+}