Apply fixes from angular v5.0.4

src/common-http/src/xhr.ts

@@ -190,16 +190,27 @@ export class HttpXhrBackend implements HttpBackend {
 
 				// Check whether the body needs to be parsed as JSON (in many cases the browser
 				// will have done that already).
-				if (ok && req.responseType === 'json' && typeof body === 'string') {
-					// Attempt the parse. If it fails, a parse error should be delivered to the user.
+				if (req.responseType === 'json' && typeof body === 'string') {
+					// Save the original body, before attempting XSSI prefix stripping.
+					const originalBody = body;
 					body = body.replace(XSSI_PREFIX, '');
 					try {
-						body = JSON.parse(body);
+						// Attempt the parse. If it fails, a parse error should be delivered to the user.
+						body = body !== '' ? JSON.parse(body) : null;
 					} catch (error) {
-						// Even though the response status was 2xx, this is still an error.
-						ok = false;
-						// The parse error contains the text of the body that failed to parse.
-						body = { error, text: body } as HttpJsonParseError;
+						// Since the JSON.parse failed, it's reasonable to assume this might not have been a
+						// JSON response. Restore the original body (including any XSSI prefix) to deliver
+						// a better error response.
+						body = originalBody;
+
+						// If this was an error request to begin with, leave it as a string, it probably
+						// just isn't JSON. Otherwise, deliver the parsing error to the user.
+						if (ok) {
+							// Even though the response status was 2xx, this is still an error.
+							ok = false;
+							// The parse error contains the text of the body that failed to parse.
+							body = { error, text: body } as HttpJsonParseError;
+							}
 					}
 				}