Allow Task.group with 0 and 1 arguments. (#2056)

lib/core/task.toit

@@ -90,16 +90,36 @@ interface Task:
     return also throws.
 
   If $required is less than the number of $lambdas, the
-    method returns when $required tasks have completed.
+    method returns when $required tasks have completed. If $required
+    is equal to 0, the method returns immediately.
+
+  # Examples
+  ```
+  results := Task.group [
+    :: 42,
+    :: 87,
+  ]
+  print results  // => {0: 42, 1: 87}.
+
+  results = Task.group --required=1 [
+    // Due to the required=1, this lambda will be aborted, once
+    // the lambda returning 42 has finished.
+    :: sleep --ms=1_000; 87,
+    :: 42,
+  ]
+  print results  // => { 1: 42 }.
+  ```
   */
   static group lambdas/List -> Map
       --required/int=lambdas.size:
     count ::= lambdas.size
     tasks ::= Array_ count
     results ::= {:}
-    if count < 2 or not (1 <= required <= count):
+    if not (0 <= required <= count):
       throw "Bad Argument"
 
+    if required == 0: return results
+
     is-stopping/bool := false
     is-canceled/bool := false
     caught/Exception_? := null

tests/task-group-test.toit

@@ -31,20 +31,27 @@ run-case lambda/Lambda -> Task:
   return child
 
 test-simple:
+  expect-structural-equals {:} (Task.group [])
+
+  expect-structural-equals { 0: 42 } (Task.group [
+    :: 42,
+  ])
+
   expect-structural-equals { 0: 42, 1: 87 } (Task.group [
     :: 42,
     :: 87,
   ])
 
 test-bad-arguments:
-  expect-throw "Bad Argument": (Task.group [])
-  expect-throw "Bad Argument": (Task.group [ :: 42 ])
   expect-throw "Bad Argument": (Task.group --required=-1 [ :: 42, :: 87 ])
-  expect-throw "Bad Argument": (Task.group --required= 0 [ :: 42, :: 87 ])
   expect-throw "Bad Argument": (Task.group --required= 3 [ :: 42, :: 87 ])
   expect-throw "Bad Argument": (Task.group --required= 9 [ :: 42, :: 87 ])
 
 test-required:
+  expect-structural-equals {:} (Task.group --required=0 [
+    :: 42,
+    :: sleep --ms=1_000; 87,
+  ])
   expect-structural-equals { 0: 42 } (Task.group --required=1 [
     :: 42,
     :: sleep --ms=1_000; 87,