log: Do some logging only in debug app, not in tests

lib/api/core.dart

@@ -1,8 +1,9 @@
 import 'dart:convert';
 
-import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 
+import '../log.dart';
+
 class Auth {
   Auth({required this.realmUrl, required this.email, required this.apiKey})
    : assert(realmUrl.query.isEmpty && realmUrl.fragment.isEmpty);
@@ -74,7 +75,7 @@ class LiveApiConnection extends ApiConnection {
     assert(_isOpen);
     final url = auth.realmUrl.replace(
         path: "/api/v1/$route", queryParameters: encodeParameters(params));
-    if (kDebugMode) print("GET $url");
+    assert(debugLog("GET $url"));
     final response = await _client.get(url, headers: _headers());
     if (response.statusCode != 200) {
       throw Exception("error on GET $route: status ${response.statusCode}");

lib/log.dart

@@ -0,0 +1,32 @@
+
+/// Whether [debugLog] should do anything.
+///
+/// This has an effect only in a debug build.
+bool debugLogEnabled = false;
+
+/// Print a log message, if debug logging is enabled.
+///
+/// In a debug build, if [debugLogEnabled] is true, this prints the given
+/// message to the log.  Otherwise it does nothing.
+///
+/// Typically we set [debugLogEnabled] so that this will print when running
+/// the app in a debug build, but not when running tests.
+///
+/// Call sites of this function should be enclosed in `assert` expressions, so
+/// that any interpolation to construct the message happens only in debug mode.
+/// To help make that convenient, this function always returns true.
+///
+/// Example usage:
+/// ```dart
+///   assert(debugLog("Got frobnitz: $frobnitz"));
+/// ```
+bool debugLog(String message) {
+  assert(() {
+    // TODO(log): make it convenient to enable these logs in tests for debugging a failing test
+    if (debugLogEnabled) {
+      print(message); // ignore: avoid_print
+    }
+    return true;
+  }());
+  return true;
+}

lib/main.dart

@@ -1,7 +1,12 @@
 import 'package:flutter/material.dart';
 
+import 'log.dart';
 import 'widgets/app.dart';
 
 void main() {
+  assert(() {
+    debugLogEnabled = true;
+    return true;
+  }());
   runApp(const ZulipApp());
 }