Handle adb error cases more gracefully

We now print a sensible message if we can't find `dart` or `adb`. Also, we
print a sensible message if the device isn't authorized.

Fixes #380
Fixes #358

bin/flutter

@@ -3,6 +3,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+set -e
+
 export FLUTTER_ROOT=$(dirname $(dirname "${BASH_SOURCE[0]}"))
 FLUTTER_TOOLS_DIR="$FLUTTER_ROOT/packages/flutter_tools"
 SNAPSHOT_PATH="$FLUTTER_ROOT/bin/cache/flutter_tools.snapshot"
@@ -20,6 +22,8 @@ if [ ! -f "$SNAPSHOT_PATH" ] || [ ! -f "$STAMP_PATH" ] || [ `cat "$STAMP_PATH"`
   echo -n $REVISION > "$STAMP_PATH"
 fi
 
+set +e
+
 $DART "$SNAPSHOT_PATH" "$@"
 
 # The VM exits with code 253 if the snapshot version is out-of-date.
@@ -29,5 +33,7 @@ if [ $EXIT_CODE != 253 ]; then
   exit $EXIT_CODE
 fi
 
+set -e
+
 $DART --snapshot="$SNAPSHOT_PATH" --package-root="$FLUTTER_TOOLS_DIR/packages" "$SCRIPT_PATH"
 $DART "$SNAPSHOT_PATH" "$@"

packages/flutter_tools/lib/src/device.dart

@@ -505,6 +505,14 @@ class AndroidDevice extends Device {
   static List<AndroidDevice> getAttachedDevices([AndroidDevice mockAndroid]) {
     List<AndroidDevice> devices = [];
     String adbPath = (mockAndroid != null) ? mockAndroid.adbPath : _getAdbPath();
+
+    try {
+      runCheckedSync([adbPath, 'version']);
+    } catch (e) {
+      _logging.severe('Unable to find adb. Is "adb" in your path?');
+      return devices;
+    }
+
     List<String> output = runSync([adbPath, 'devices', '-l']).trim().split('\n');
 
     // 015d172c98400a03       device usb:340787200X product:nakasi model:Nexus_7 device:grouper
@@ -514,6 +522,8 @@ class AndroidDevice extends Device {
     // 0149947A0D01500C       device usb:340787200X
     RegExp deviceRegex2 = new RegExp(r'^(\S+)\s+device\s+\S+$');
 
+    RegExp unauthorizedRegex = new RegExp(r'^(\S+)\s+unauthorized$');
+
     // Skip first line, which is always 'List of devices attached'.
     for (String line in output.skip(1)) {
       // Skip lines like:
@@ -539,6 +549,13 @@ class AndroidDevice extends Device {
         Match match = deviceRegex2.firstMatch(line);
         String deviceID = match[1];
         devices.add(new AndroidDevice(id: deviceID));
+      } else if (unauthorizedRegex.hasMatch(line)) {
+        Match match = unauthorizedRegex.firstMatch(line);
+        String deviceID = match[1];
+        _logging.warning(
+          'Device $deviceID is not authorized.\n'
+          'You might need to check your device for an authorization dialog.'
+        );
       } else {
         _logging.warning(
           'Unexpected failure parsing device information from adb output:\n'