Improve test error messages from flutter

For some reason the error message from flutter SDK might contain no useful information. Basically it just says that test failed and you should see the logs. Logs itself are provided as content of `print` event. This commit adds special processing for this behavior - it parses actual error message out of print event.

__tests__/__snapshots__/dart-json.test.ts.snap

@@ -160,8 +160,11 @@ The test description was:
   pass updateShouldNotify
 ════════════════════════════════════════════════════════════════════════════════════════════════════",
                 "line": 112,
-                "message": "Test failed. See exception logs above.
-The test description was: pass updateShouldNotify",
+                "message": "The following TestFailure object was thrown running a test:
+  Expected: <2>
+  Actual: <1>
+Unexpected number of calls
+",
                 "path": "test/value_listenable_provider_test.dart",
               },
               "name": "pass updateShouldNotify",

src/parsers/dart-json/dart-json-parser.ts

@@ -161,14 +161,14 @@ export class DartJsonParser implements TestParser {
     }
 
     const {trackedFiles} = this.options
-    const message = test.error?.error ?? ''
     const stackTrace = test.error?.stackTrace ?? ''
     const print = test.print
       .filter(p => p.messageType === 'print')
       .map(p => p.message)
       .join('\n')
     const details = [print, stackTrace].filter(str => str !== '').join('\n')
     const src = this.exceptionThrowSource(details, trackedFiles)
+    const message = this.getErrorMessage(test.error?.error ?? '', print)
     let path
     let line
 
@@ -191,6 +191,21 @@ export class DartJsonParser implements TestParser {
     }
   }
 
+  private getErrorMessage(message: string, print: string): string {
+    if (this.sdk === 'flutter') {
+      const uselessMessageRe = /^Test failed\. See exception logs above\.\nThe test description was:/m
+      const flutterPrintRe = /^══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞═+\s+(.*)\s+When the exception was thrown, this was the stack:/ms
+      if (uselessMessageRe.test(message)) {
+        const match = print.match(flutterPrintRe)
+        if (match !== null) {
+          return match[1]
+        }
+      }
+    }
+
+    return message || print
+  }
+
   private exceptionThrowSource(ex: string, trackedFiles: string[]): {path: string; line: number} | undefined {
     const lines = ex.split(/\r?\n/g)