Nicer error message for TLS errors (#14285)

Also some trivial improvements to style.

packages/flutter_tools/lib/src/base/io.dart

@@ -42,6 +42,7 @@ export 'dart:io'
         // File              NO! Use `file_system.dart`
         // FileSystemEntity  NO! Use `file_system.dart`
         GZIP,
+        HandshakeException,
         HttpClient,
         HttpClientRequest,
         HttpClientResponse,

packages/flutter_tools/lib/src/base/net.dart

@@ -37,7 +37,18 @@ Future<List<int>> _attempt(Uri url) async {
   } else {
     httpClient = new HttpClient();
   }
-  final HttpClientRequest request = await httpClient.getUrl(url);
+  HttpClientRequest request;
+  try {
+    request = await httpClient.getUrl(url);
+  } on HandshakeException catch (error) {
+    printTrace(error.toString());
+    throwToolExit(
+      'Could not authenticate download server. You may be experiencing a man-in-the-middle attack,\n'
+      'your network may be compromised, or you may have malware installed on your computer.\n'
+      'URL: $url',
+      exitCode: kNetworkProblemExitCode,
+    );
+  }
   final HttpClientResponse response = await request.close();
   if (response.statusCode != 200) {
     if (response.statusCode > 0 && response.statusCode < 500) {

packages/flutter_tools/lib/src/doctor.dart

@@ -199,10 +199,13 @@ class _FlutterValidator extends DoctorValidator {
     final FlutterVersion version = FlutterVersion.instance;
 
     messages.add(new ValidationMessage('Flutter version ${version.frameworkVersion} at ${Cache.flutterRoot}'));
-    if (Cache.flutterRoot.contains(' '))
+    if (Cache.flutterRoot.contains(' ')) {
       messages.add(new ValidationMessage.error(
-        'Flutter SDK install paths with spaces are not yet supported. (https://github.com/flutter/flutter/issues/6577)\n'
-        'Please move the SDK to a path that does not include spaces.'));
+        'Flutter SDK install paths with spaces are not yet supported. '
+        '(https://github.com/flutter/flutter/issues/6577)\n'
+        'Please move the SDK to a path that does not include spaces.'
+      ));
+    }
     messages.add(new ValidationMessage(
       'Framework revision ${version.frameworkRevisionShort} '
       '(${version.frameworkAge}), ${version.frameworkDate}'
@@ -215,13 +218,16 @@ class _FlutterValidator extends DoctorValidator {
 
     // Check that the binaries we downloaded for this platform actually run on it.
     if (!_genSnapshotRuns(genSnapshotPath)) {
-      messages.add(new ValidationMessage.error('Downloaded executables cannot execute '
-          'on host (see https://github.com/flutter/flutter/issues/6207 for more information)'));
+      messages.add(new ValidationMessage.error(
+        'Downloaded executables cannot execute '
+        'on host (see https://github.com/flutter/flutter/issues/6207 for more information)'
+      ));
       valid = ValidationType.partial;
     }
 
     return new ValidationResult(valid, messages,
-      statusInfo: 'on ${os.name}, locale ${platform.localeName}, channel ${version.channel}');
+      statusInfo: 'on ${os.name}, locale ${platform.localeName}, channel ${version.channel}'
+    );
   }
 }
 

packages/flutter_tools/lib/src/services.dart

@@ -78,7 +78,7 @@ Future<String> getServiceFromUrl(String url, String rootDir, String serviceName)
   if (url.startsWith('android-sdk:') && androidSdk != null) {
     // It's something shipped in the standard android SDK.
     return url.replaceAll('android-sdk:', '${androidSdk.directory}/');
-  } else if (url.startsWith('http')) {
+  } else if (url.startsWith('http:') || url.startsWith('https:')) {
     // It's a regular file to download.
     return await cache.getThirdPartyFile(url, serviceName);
   } else {