be more lenient when looking for matching android sdk components (#3830)

* be more lenient when looking for matching android sdk components

* use a null aware operator

packages/flutter_tools/lib/src/android/android_sdk.dart

@@ -147,10 +147,7 @@ class AndroidSdk {
         .toList();
     }
 
-    // Here we match up platforms with cooresponding build-tools. If we don't
-    // have a match, we don't return anything for that platform version. So if
-    // the user only has 'android-22' and 'build-tools/19.0.0', we don't find
-    // an Android sdk.
+    // Match up platforms with the best cooresponding build-tools.
     _sdkVersions = platforms.map((String platformName) {
       int platformVersion;
 
@@ -167,6 +164,8 @@ class AndroidSdk {
         return version.major == platformVersion;
       }).toList());
 
+      buildToolsVersion ??= Version.primary(buildTools);
+
       if (buildToolsVersion == null)
         return null;