Add simple depends_on API.

recipes/list_primes.py

+from recipe_engine.recipe_api import Property
+from recipe_engine import config
+import math
+
+# This recipe computes all the primes up to the value up_to.
+
+DEPS = [
+    'recipe_engine/properties',
+]
+
+PROPERTIES = {
+    'up_to': Property(kind=int, default=50),
+}
+
+RETURN_SCHEMA = config.ReturnSchema(
+    result=config.List(int)
+)
+
+def is_prime(val):
+  if val % 2 == 0:
+    return False
+  for div in range(3, math.ceil(math.sqrt(val))):
+    if val % div == 0:
+      return False
+  return True
+
+def RunSteps(api, up_to):
+  all_primes = []
+  prime = 2
+  while prime < up_to:
+    all_primes.append(prime)
+    res = api.depend_on('next_prime', {'curr_prime': prime})
+    prime = res.result
+    if not is_prime(prime):
+      raise api.StepFailure("OH NOES")
+
+  return RETURN_SCHEMA(result=all_primes)
+
+def GenTests(api):
+  yield (
+      api.test('basic') +
+      api.properties(up_to=3) +
+      api.depend_on('next_prime', {'result': 3})
+  )
+
+  yield (
+      api.test('failure') +
+      api.properties(up_to=10) +
+      api.depend_on('next_prime', {'result': 4})
+  )